I have been working for 3 years through a temp. agency for a N. Carolina university. 3 years with no benefits, vacation pay, sick leave, retirement, etc. I have been told by co-workers that after 90 days of continous employment in the same job it is required by law that I be hired full-time. Is this true?

No, it is not.

So I have no choice but to hold out for a position or to look elsewhere?

That's right.